1. Field of The Invention
The present invention relates generally to a control system for an automatic transmission of an automotive vehicle. More particularly, the invention is directed to an automatic transmission control system which is operable to hold forward speed even when a driver mistakenly shifts a selector lever to a reverse range while a vehicle is traveling forward.
2. Description of The Prior Art
Japanese Patent First Publication No. 1-120467 discloses a control system for an automatic transmission which is capable of preventing the transmission from shifting to a reverse mode when a selector lever is mistakenly moved to a reverse position for avoiding the transmission from breaking down. This conventional transmission control system includes a directional control valve which is arranged in a hydraulic line communicating between a manual selector valve and a reverse frictional element serving to establish the reverse mode. When a sensor signal from a vehicle speed sensor indicates that the vehicle is traveling forward, the directional control valve assumes a position which drains hydraulic pressure in the reverse frictional element to release it. Alternatively, when the vehicle is not traveling forward, the directional control valve is placed in a position which directs hydraulic pressure from the manual selector valve to the reverse frictional element to enable the reverse mode. In this manner, even when the selector lever is mistakenly moved to the reverse position, the transmission is prevented from shifting to the reverse mode.
The above prior art transmission control system, however, encounters a drawback in that the system requires an additional solenoid operated valve for switching a position of the directional control valve. This consumes valuable space and increases the number of parts, resulting in the total costs of the system being increased.